RF antennas for transmission of electromagnetic signals are preferably designed to have a size of at least one quarter of the wavelength of the transmitted signal, since this generally allows high antenna efficiency, wide bandwidth and substantially real input impedance. However, many apparatuses do not have room for an antenna large enough to satisfy this condition. For an RF signal with a frequency of e.g. 100 MHz, one quarter of the wavelength equals 0.75 m. It is thus common to utilise antennas that are considerably smaller than one quarter of the wavelength. Such antennas are generally referred to as “electrically short” or “electrically small” antennas. Electrically short antennas inherently exhibit low radiation resistance and low efficiency. Their efficiency may be increased by reducing resistive losses in the antenna and the associated circuits, which, however, increases the quality factor (Q) of the antenna so that the bandwidth decreases. At a typical quality factor of 50, the 3-dB bandwidth of the antenna is 2% of the centre frequency.
The co-pending patent application EP 11 184 079.9 and the corresponding provisional patent application U.S. 61/543,821 disclose a transmitter for transmitting an electromagnetic signal via an electrically short antenna. The transmitter comprises a reactive energy storage, which forms a resonance circuit with the antenna. The transmitter further comprises a number of output stages, which provide an electric transmission signal to the resonance circuit. The electric transmission signal is frequency- and/or phase-modulated with an information signal and thus has an instantaneous frequency that varies in dependence on the information signal. In order to allow efficient transmission of electromagnetic signals with a bandwidth exceeding that of the resonance circuit, the transmitter dynamically changes the resonance frequency of the resonance circuit by changing the effective reactance of the energy storage in dependence on the information signal. The electronic switch elements used for changing the effective reactance of the energy storage must be protected against high voltages occurring at the antenna terminals. Therefore, the electronic switch elements are not connected directly to the antenna terminals but instead indirectly via voltage dividers formed by serially connected capacitors. The outputs of the individual output stages are similarly connected to the antenna terminals via capacitors forming impedance transformers yielding higher signal voltages at the antenna terminals than at the outputs of the output stages.
In an embodiment disclosed in the above mentioned co-pending patent applications, the transmitter further comprises a receiver circuit connected to the resonance circuit, which allows the transmitter to function as a half-duplex transceiver. In the receive mode, the transmitter can, however, not take advantage of a dynamic change of the resonance frequency of the resonance circuit, since this would require knowledge of the instantaneous frequency of the received signal before receiving it, and this information is obviously only available after receiving the signal. An alternative method for temporarily increasing the bandwidth of the resonance circuit and thus of the receiver is to temporarily add resistive loads to the antenna terminals, e.g. by means of electronic switch elements. This method is not disclosed in the above mentioned applications, but is generally known in the prior art, e.g. from patent application FR 2 911 805. The same method could also be used for temporarily increasing the bandwidth of the resonance circuit during transmission, e.g. in order to allow transmission of signals with a bandwidth exceeding the bandwidths achievable by the dynamic change of the resonance frequency of the resonance circuit. However, adding resistive loads directly to the antenna terminals by means of switches would subject the switches to possibly large signal voltages occurring across the antenna terminals, which could damage the switches or shorten their life-time.